FAILED: patch "[PATCH] mm/damon/ops-common: use nr_accesses moving sum for quota" failed to apply to 6.6-stable tree

FAILED: patch "[PATCH] mm/damon/ops-common: use nr_accesses moving sum for quota" failed to apply to 6.6-stable tree

[gregkh]

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 6c25083f7ae7e2660d766169e5b4d3e96010503f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026090847-trapdoor-essence-0783@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 6c25083f7ae7e2660d766169e5b4d3e96010503f Mon Sep 17 00:00:00 2001
From: SJ Park <sj@kernel.org>
Date: Sun, 19 Jul 2026 09:11:33 -0700
Subject: [PATCH] mm/damon/ops-common: use nr_accesses moving sum for quota
 score

Since commit 42f994b71404 ("mm/damon/core: implement scheme-specific apply
interval"), DAMOS scheme can be applied at any time.  At that time,
nr_accesses may not be fully aggregated.  But the quota prioritization
score is calculated using the not fully aggregated count.  As a result,
the performance of DAMOS could be degraded.  Fix by using
damon_nr_accesses_mvsum() instead.

The user impact of the issue is suboptimum DAMOS performance under certain
setups.  Nonetheless, the bug was there from the beginning of the setup
availability.  In other words, the suboptimum performance is the baseline
of the setup and hence it didn't cause regression.  Also the extent of the
suboptimality was not big enough to be found from users and testers.
Still, this is a clear bug that is better to be fixed, and can be easily
fixed.

Link: https://lore.kernel.org/20260719161136.90191-1-sj@kernel.org
Fixes: 42f994b71404 ("mm/damon/core: implement scheme-specific apply interval")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.7.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index e59f5268d503..d7d7f100389b 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -111,8 +111,9 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
 	unsigned int age_weight = s->quota.weight_age;
 	int hotness;
 
-	freq_subscore = r->nr_accesses * DAMON_MAX_SUBSCORE /
-		damon_nr_samples_per_aggr(&c->attrs);
+	freq_subscore = mult_frac(damon_nr_accesses_mvsum(r, c),
+			DAMON_MAX_SUBSCORE,
+			damon_nr_samples_per_aggr(&c->attrs));
 
 	age_in_sec = (unsigned long)r->age * c->attrs.aggr_interval / 1000000;
 	if (age_in_sec)

Re: FAILED: patch "[PATCH] mm/damon/ops-common: use nr_accesses moving sum for quota" failed to apply to 6.6-stable tree

[SJ Park]

On Tue, 08 Sep 2026 14:52:47 +0200 <gregkh@linuxfoundation.org> wrote:

> 
> The patch below does not apply to the 6.6-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.
[...]
> ------------------ original commit in Linus's tree ------------------
> 
> From 6c25083f7ae7e2660d766169e5b4d3e96010503f Mon Sep 17 00:00:00 2001
> From: SJ Park <sj@kernel.org>
> Date: Sun, 19 Jul 2026 09:11:33 -0700
> Subject: [PATCH] mm/damon/ops-common: use nr_accesses moving sum for quota
>  score
> 
> Since commit 42f994b71404 ("mm/damon/core: implement scheme-specific apply
> interval"), DAMOS scheme can be applied at any time.  At that time,
> nr_accesses may not be fully aggregated.  But the quota prioritization
> score is calculated using the not fully aggregated count.  As a result,
> the performance of DAMOS could be degraded.  Fix by using
> damon_nr_accesses_mvsum() instead.
> 
> The user impact of the issue is suboptimum DAMOS performance under certain
> setups.  Nonetheless, the bug was there from the beginning of the setup
> availability.  In other words, the suboptimum performance is the baseline
> of the setup and hence it didn't cause regression.  Also the extent of the
> suboptimality was not big enough to be found from users and testers.
> Still, this is a clear bug that is better to be fixed, and can be easily
> fixed.
> 
> Link: https://lore.kernel.org/20260719161136.90191-1-sj@kernel.org
> Fixes: 42f994b71404 ("mm/damon/core: implement scheme-specific apply interval")

Seems the broken commit is backported to 6.6.y even though it is not a fix,
maybe to resolve a conflict of another fix.  That should be why this patch is
tried to be applied on 6.6.y.

Though the broken commit indeed introduced the feature to DAMON API, the commit
didn't introduce any DAMON API caller that uses the feature.  Apparently commit
a2a9f68e358f ("mm/damon/sysfs-schemes: support DAMOS apply interval") has
further integrated the feature with DAMON sysfs interface, and exposed it to
users.  So technically speaking, it is the real broken commit.

And the real broken commit is not in 6.6.y.  So, this patch is not really
needed for 6.6.y.


Thanks,
SJ

[...]